This invention relates to a process for the preparation of carnosic acid from rosemary or sage. The invention also relates to the use of carnosic acid for its anticarcinogenic and antiviral properties.
Carnosic acid is a phenolic diterpene which corresponds to the empirical formula C.sub.20 H.sub.28 O.sub.4 and which has the following structure ##STR1## It is a constituent of the species Salvia and Rosmarinus where it is mainly to be found in the leaves. It was discovered for the first time by Linde in Salvia officinalis [Helv. Chim Acta 47, 1234 (1962)] and by Wenkert et al. in Rosmarinus officinalis [J. Org. Chem. 30, 2931 (1965)]. It was then positively identified in various other species of sage, such as for example Salvia canariensis [Savona and Bruno, J. Nat. Prod. 46, 594 (1983)] or Salvia willeana [de la Torre et al., Phytochemistry 29, 668 (1990)]. It is also present in Salvia triloba and Salvia sclarea.
Carnosic acid is a powerful antioxidant [Brieskorn and Domling, Z. Lebensm. Unters. Forsch. 141, 10 (1969)] and, according to a number of Russian works where it bears the name salvine, an antibiotic against Staphylococcus aureus [CA 86, 117603r; 90, 49011b; 97, 67513r, 69163a, 69164b; 104, 221930w; 111, 130594t] and against certain microorganisms responsible for dental caries and bad breath [CA 97, 84835q]. In connection with this latter property, it is mentioned in the prior art for the production of dentifrices and mouthwashes [JP 59 103 665, Lion Corp.].
Despite this large of number of references, the isolation on a preparative scale of carnosic acid from rosemary or sage has never been described either by Linde or Wenkert, where its existence is indirectly proved, or by those whose subsequently identified it in various species of sage.
By contrast, many other phenolic diterpenes similar in structure to carnosic acid have been isolated from one or other of the two species Salvia and Rosmarinus. They include carnosol [Brieskorn et al., Chem. Ber. 95, 3034 (1962); J. Org. Chem. 29, 2293 (1964)] and, more recently, rosmanol [Inatani et al., Agric. Biol. Chem. 46, 1661 (1982)] or rosmaridiphenol [Houlihan et al., J. Am. Oil Chem. Soc. 61, 1036 (1984)], the last two having been patented as new antioxidant principles of rosemary U.S. Pat. No. 4,450,097, Nakatani et al., Lion Corp., Tokyo; U.S. Pat. No. 4,638,095, Chang et al., Research Corp., N.Y.].
Although all these compounds are worthy of interest as antioxidants, it nevertheless remains that, compared with carnosic acid, they have certain unfavourable features. Their content in rosemary or sage is much lower than that of carnosic acid.
Dried leaves of rosemary or sage (species Salvia officinalis) contain between 1.5 and 2.5% carnosic acid and only about 0.3-0.4% carnosol. Rosmanol and rosmaridiphenol are present in undetectable concentrations. Accordingly, from the point of view of the economy of a production process, carnosic acid has an indisputable advantage. According to the data disclosed in U.S. Pat. No. 4,450,097 for example, it may be calculated that the yield of rosmanol isolated from rosemary is only 0.01%.
As was demonstrated by Wenkert et al., carnosol is an oxidative artefact of carnosic acid. This oxidation takes place in the presence of oxygen both after the harvesting of rosemary or sage in the leaves left to dry in air (it can incidentally be demonstrated that the freshly cut leaves of rosemary do not contain carnosol) and when the leaves are subjected to extraction with solvents or when the extracts themselves are subjected to conventional operations of fractionation, enrichment and purification. There is every reason to assume that rosmanol, which has been identified in a rosemary fraction subjected to an alkaline treatment, is itself a subsequent product of the oxidation of carnosic acid, as Wenkert et al. already suggested; the same may also be reasonably assumed of rosmaridiphenol. Carnosic acid is therefore the only phenolic diterpene present in the native state in rosemary and sage and, accordingly, has the sole right to be called a natural product.
Some methods for the preparation of carnosic acid by chemical synthesis have also been proposed in the literature by W. L. Meyer et al. [Tetrahedron Letters 1966, 4261; 1968, 2963; J. Org. Chem. 41, 1005 (1976)]. However, the syntheses involved are long and complex and, for economic reasons, cannot be applied to an industrial process. In addition, these syntheses lead to racemic mixtures of carnosic acid precursors and not to the pure enantiomers. It should also be pointed out that these works stop at the preparation of carnosic acid precursors and omit to describe the final preparation step(s). Another method of obtaining carnosic acid has been described in the literature by Brieskorn and Domling [Arch. Pharm. 302, 641 (1969)], comprising the catalytic reduction of carnosol. Once again, the application of this process on a large scale could not be envisaged on account of the non-availability of carnosol.